1. Field of the Invention
This invention relates generally to plasma torches for cutting and welding, and in particular, to a dual flow nozzle shield which permits primary gas to pass both through an orifice in the center of the nozzle shield and a portion of the gas flow back up and over the outside of the shield via a plurality of spaces around the shield perimeter, the bypass gas flow being directed toward an arc to block splattering molten metal.
2. Description of Related Art
The basic elements or typical plasma-arc torch comprises a torch barrel, an electrode mounted within the body, a nozzle with a center orifice that produces a pilot arc to the electrode to initiate a plasma-arc in a flow of a suitable gas, tubes extending from the rear of the barrel for receiving gas line passages in the torch for cooling, and a ceramic insert mounted at the face of the torch immediately adjacent to a workpiece.
During the piercing and cutting operation molten metals are inclined to fly upward toward the nozzle or shield in front of a nozzle which degrades the function of these components and cutting operation. Several approaches in the prior art have been tried to keep splattering molten metals from flying into the shield of nozzle.
In U.S. Pat. No. 4,861,962, issued Aug. 29, 1989 to Sanders et al. and assigned to Hypertherm, Inc., a nozzle shield is described for a plasma-arc torch operating in the 0-200 ampere range having a shield mounted at its lower end adjacent to a workpiece to block splattered molten metal from reaching a nozzle of the torch. The shield is electrically insulated by mounting it on an insulating ring. A secondary gas flow through the torch passes through the space between the nozzle and the shield to provide cooling. A first portion of the secondary gas flow exits through at least one bleed port and a second portion exits through said shield exit orifice and is of a velocity to stabilize the plasma produced by primary gas flow exiting the torch of the nozzle orifice 18 and the shield exit orifice.
In U.S. Pat. No. 5,120,930, issued Jun. 9, 1992 to Sanders et al. and assigned to Hypertherm, Inc., a plasma-arc torch is described with improved nozzle shield and step flow. This patent is a continuation-in-part of U.S. Pat. No. 4,861,962 (described above) but further claims that the secondary gas flow means includes at least one opening in the shield in fluid communication with the space between the nozzle and the shield and located before the exit orifice to bleed off a first portion of the secondary gas flow, at least one opening being angled from the vertical at an angle greater than zero degrees. A second portion of the secondary gas flow exits through the shield exit orifice and being of a velocity to stabilize the plasma produced by the primary gas flow exiting the torch of the nozzle orifice and the shield exit orifice.
In U.S. Pat. No. 5,132,512, issued Jul. 21, 1992 to Sanders et al. and assigned to Hypertherm, Inc., an arc torch nozzle shield mounted on a torch body is described, the shield generally surrounding the nozzle in a spaced relationship and having an exit orifice aligned with the nozzle orifice, means for insulating the shield electrically from the body to prevent double arcing and means for producing secondary gas flow through the body, the secondary gas flow passing through the space between the nozzle and the shield at a rate sufficient to cool the shield, the secondary gas flow means including at least one opening in the shield in fluid communication with the space and located before the exit orifice to bleed off a portion of the secondary gas flow, and at least one opening being angled from the vertical at an angle greater than zero degrees.
In U.S. Pat. No. 5,591,357, issued Jan. 7, 1997 to Couch, Jr. et al. and assigned to Hypertherm, Inc., a plasma-arc torch having a secondary gas flow is described that is extremely large during piercing of a workpiece to keep splattered molten metal away from the torch and thereby prevent double arcing. A nozzle is mounted immediately below an electrode in a spaced relationship to define a plasma-arc chamber therebetween where plasma gas fed from a swirl ring is ionized to form either a pilot arc or plasma jet between the electrode and a workpiece. The secondary flow path including the orifice, prechamber and the swirl ring are the principal features of this invention. The swirl ring contains a set of off-center, or canted holes which introduce a swirling movement to the flow which facilitates the interaction of the secondary gas stream with the jet. The claims are directed to a method of operating a plasma-arc cutting system comprising the steps of directing a plasma gas flow to a plasma chamber, forming a secondary gas flow as a mixture of non-oxidizing gas and at least 40% of oxidizing gas, directing the secondary gas flow from an inlet to a flow path, altering the secondary gas flow, in the secondary gas flow path, and directing the secondary gas flow from a secondary gas flow path through secondary gas flow exit orifice and onto the plasma-arc as the plasma-arc passes through file nozzle exit orifice.
In U.S. Pat. No. 5,614,110, issued on Mar. 25, 1997 to Shintani et al. and assigned to Komatso, Ltd., of Tokyo, Japan, a method and apparatus for varying protective gas composition between piercing and cutting with a plasma torch is described so that it is possible to protect a nozzle by only a small amount of dross being blown upward. The plasma torch comprises an electrode, a nozzle having an orifice, a plasma gas passage, a protective cap having an opening in alignment with the orifice and a protective gas passageway. A piercing completion detection unit detects an electronic current between an electrode and a workpiece at the time of piercing and outputs a signal at the time of the completion of the piercing. A flow regulator is provided in a protective gas circuit which switches, in response to the piercing completion signal, the flow rate of the protective gas from a high flow rate at the time of piercing to a low flow rate at the time of cutting. An expensive gas such as hydrogen or argon can be used as a protective gas for obtaining a good cut surface quality. A low-price protective gas can be used at the time of piercing.